A spatial filter is used with an image pickup tube for converting an input light image into an electric signal or with a photographic system which consists in projecting light only once on a monochrome film and thereafter reproducing from said film an image having the original colors of a foreground subject.
An image pickup tube 11 is generally constructed as illustrated in FIG. 1. An input light image passing through an optical system 17 is further conducted through a spatial filter 13 formed on the inner wall of a faceplate 12 and a transparent conductive layer 14 and then to a photoconductive layer 15, thereby producing a potential image on said photoconductive layer 15. When an image on the photoconductive layer 15 is scanned by electron beams discharged from an electron gun 16 provided at the other end of the image pickup tube, then the potential of said potential image is brought back to the original level and current corresponding to said variations of potential is generated as an electric signal. In recent years, a single- or double- tube type color image pickup device is being developed which is provided with one or two image pickup tubes.
There will now be described a spatial filter used with, for example, a single tube type color image pickup device. A spatial filter is generally prepared by arranging yellow-permeable filter stripes and cyan-permeable filter stripes on a transparent substrate made of, for example, glass at a prescribed interval and in a state intersecting each other at a prescribed angle. Both types of filter stripe are formed by photoetching, namely, evaporating metal such as silver or nickel on a transparent substrate made of, for example, glass, coating photoresist on said evaporated metal layer, projecting light on said coating, followed by development, thereby producing a pattern. The exposed metal layer is partly etched off by an etching solution and the photoresist is removed. As the result, the metal is left on the substrate in the form of stripes to be used as a mask for the subsequent evaporation of filter stripes. A yellow-permeable filter layer is evaporated uniformly on the metal mask as well as on those parts of the substrate surface on which said metal mask is not formed. When, thereafter, said metal mask is etched off, desired yellow-permeable filter stripes are obtained. Cyan-permeable filter stripes intersecting the yellow-permeable filter stripes are formed by repeating the same operation as described above. Those parts of the surface of the substrate which are and are not coated with filter stripes thus formed are uniformly covered first with a transparent conductive layer and then with a photoconductive layer, providing an image pickup tube target. The spatial filter is so disposed in the image pickup tube as to allow the substrate to constitute the faceplate of said image pickup tube.
Where, however, the spatial filter is integrally formed with the image pickup tube target, the transparent conductive layer and photoconductive layer are evaporated with surface irregularities on the filter stripes and on those parts of the surface of the substrate on which the filter stripes are not formed. Such surface irregularities prevent some portions of the transparent conductive layer and photoconductive layer from being deposited on the filter stripes, resulting in the occurrence of defective spots from which any electric signal can not be drawn out. Or while the spatial filter is operated in the image pickup tube, an electric field is concentrated locally on the projecting parts of the surface of a photoconductive layer, namely, those parts covering the filter stripes, eventually leading to the local damage of the image pickup tube target. The prior art spatial filter has further drawbacks that metals, photoresist, etching solutions and washing liquids used in forming filter stripes remain in interstices between said filter stripes and transparent conductor layer, and these residual substances harmfully affect the photoconductive layer and give rise to undesired noises which appear in an output image in the form of white blots.